Rat populations are soaring in cities around the world as rising temperatures make it easier for the rodents to breed, according to a new study. Researchers analysed public sightings and infestation reports from 16 cities, finding that 11 showed significant increasing trends in rat numbers.
The sharpest increases were in Washington DC (390% over the past decade), San Francisco (300%), Toronto (186%) and New York City (162%). The study, published in the journal Science Advances, warns that these trends are likely to continue as global temperatures rise further.
While cities such as London and Paris were not included due to a lack of comparable data, lead researcher Jonathan Richardson from the University of Richmond said there was no reason to expect different outcomes elsewhere. “There’d be no reason to expect it to be different in other places,” he said.
In Toronto, Canada’s largest city, officials have described a “perfect rat storm”. Pest control firm Orkin reported a surge in calls, while the city helpline received 1,600 rat-related complaints in 2023, up from 940 in 2019. Two city councillors have called for a formal management plan to address the crisis.
Rising temperatures were closely linked to increasing rat numbers, the study found. Milder winters allow rats to breed for longer periods and forage more extensively. In Toronto, cold winters had previously acted as “mother nature’s pest control”, but warming conditions have enabled rodents to thrive.
Not all cities saw increases: Tokyo, Louisville and New Orleans recorded declining rat numbers, which researchers attribute to cultural norms, cleanliness standards, and public education campaigns. The study suggests that the most effective pest management strategies involve making urban environments less rat-friendly, such as securing rubbish in containers rather than leaving bags on the street.
